Examples of modulation in a Mach-Zehnder (MZ) optical modulator include dual-polarization quadrature phase-shift keying (DP-QPSK) and dual-polarization quadrature amplitude modulation (DP-16QAM). In these modulation schemes, light is modulated by a plurality of data streams through polarization multiplexing. A timing difference in data streams between polarized waves, which can be seen in the modulation, degrades signal properties. The timing difference hence needs to be reduced (c.f., Japanese Patent Application Laid-Open No. 2015-052670, i.e., Patent Document 1).
For instance, Japanese Patent Application Laid-Open No. 2011-044906 (i.e., Patent Document 2) describes a laser light beam branched into two light beams. The light beams are individually modulated, and then they are synthesized. The polarization of one of the light beams is maintained before the synthesis (X-polarized wave). The polarization of the other light beam is 90° rotated before the synthesis (Y-polarized wave). A timing difference between the polarized waves can be seen after the modulation. This is because different data streams have different distances from a data generator, which generates data streams used in modulation, to a modulator, or different distances in the inside of the modulator. The timing difference causes the degradation of the signal properties. To address such degradation, the intensity of a synthesized output consisting of the X-polarized wave and the Y-polarized wave is detected; then, the timing of each data stream is controlled using the intensity information. This reduces the timing difference in data between the polarized waves.